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Uninvited visitors to apple trees look unsightly in a yard you’ve worked hard to cultivate. Thin, patchy layers of fungi like apple scab, powdery mildew or rust (which should not be confused with common, innocuous mushroomlike fungi) can cause serious damage to trees if left untreated. Even lichen, while harmless in older trees, can be hazardous to those that are still developing. Several options exist for killing fungus and lichen on apple trees.

Copper Sulfate

Copper sulfate is readily obtained online or at most garden stores, and is easiest to use in a fluid concentrate form. Although it is safe to use to treat fungus and lichen, keep children and pets away from it during application, as it can irritate skin and eyes, and can cause vomiting, and infrequently poisoning, when ingested. Wear protective clothing, and apply a watered-down solution carefully with a paintbrush (as opposed to spraying) to avoid getting it on yourself.

Sunlight

Lichens and fungi thrive in dark, shady, sheltered areas. If your trees are in no immediate danger and you prefer a natural route, wait for the next pruning season, and prune your trees to allow significantly more sunlight to penetrate to the trunk and branches, creating an inhospitable environment. As apples grow well in these conditions anyway, you won’t end up harming either your trees or your crop for the following season.

Fungicide

Many brands of fungicide are available on the market for treating fungus on apple trees. Although none are specifically recommended for lichen treatment, they usually work because lichen is a combination of fungus with a photosynthetic partner, usually algae. Home improvement stores and online retailers can both provide a variety of appropriate fungicides; just make sure you read about them before you buy, as only a few are organic, and they range in toxicity level.

Scraping

If simple treatments or creating more sunlight does not do the trick, you may wish to scrape your trees. Affected areas that won’t take a surface-deep treatment can instead be cleansed with thorough bark removal and digging out any other wood that looks affected. Keep an eye on the tree afterward to make sure it is healing properly on its own and shows no signs of recurring fungal infection; if it does, it might require another treatment.